1. Field of the Invention
The present invention relates to the field of laser material processing methods and systems, and specifically, to scanned laser processing methods and systems for the processing of semiconductor wafers, electronic substrates, and workpieces to be laser micromachined.
2. Background Art
Conventionally, round spots have been used for a majority of precision-scanned, laser-processing applications. Many laser sources such as Nd:YAG lasers produce round Gaussian beams, which, when imaged through conventional spherical optics, produce round spots. These spots are scanned across target sites to process material, and the resulting laser-material interaction removes or otherwise alters the targeted material. In many laser-processing applications, system throughput is limited by the average power, the substrates' fluence damage threshold, and, in pulsed systems, the laser q-rate and the laser pulse characteristics.
Exemplary micromachining operations include link blowing of redundant memory circuits, laser trimming, and circuit fabrication. For processing applications such as blowing sub-micron width fuses on a memory device, efficient coupling of energy to a narrow fuse with minimum lateral and substrate damage is desirable. Large round spots may cause undesirable adjacent link damage as shown in FIG. 19a. While smaller spots allow finer pitched fuses to be processed, the potential for substrate damage can increase with the higher fluence of a decreasing spot size as shown in FIG. 19b. When micromachining a line made from a sequence of small laser spots, the spot overlap or so-called “bite size” and q-rate are two of the process characteristics that determine maximum scan velocity. Laser trimming applications requiring a wide kerf width may require multiple passes with a small spot when the peak fluence of a larger spot is inadequate. In the field of a lead frame fabrication, wherein a fine pitch lead on a large lead count device is machined, a rotating elongated spot is used as described in U.S. Pat. No. 5,632,083.
Out-of-round spots are often considered as system defects that limit process quality. Much effort has been expended in the field of laser optics to improve beam quality, to circularize beams from diode lasers, and to design and implement highly corrected optics for diffraction-limited systems. Vector diffraction effects used for beam-shape compensation are described in U.S. Pat. No. 4,397,527.
Many techniques are known for beam shaping and spot shaping. One method is a phase plate used with a round beam to modify the spot shape for processing memory fuses as shown in the upper portion of FIG. 20 and as taught by Cordingley in U.S. Pat. No. 5,300,756. The primary effect using this simple type of phase plate is to create a top-hat distribution profile as shown in the lower portion of FIG. 20. However, techniques for creating an oblong spot are also described.
Use of an anamorphic spot with dithering for shaping a laser beam intensity profile is described in U.S. Pat. No. 6,341,029. The anamorphic spot allows sharper line edges to be formed with a narrowed spot width, while an increased spot length maintains desired total power without exceeding process limits on integrated power per unit substrate area.
Veldkamp in U.S. Pat. No. 4,410,237 describes a diffraction grating and prism method for transforming a round Gaussian beam to elongated flat-top profiles.
Dickey in U.S. Pat. No. 5,864,430 describes a phase-based method for transforming a round Gaussian beam to a flat-top, square, or rectangular-shaped spot.
Yet another technique is creating an array of spots such as disclosed by James in U.S. Pat. No. 5,463,200.
Another well known technique is the imaged aperture mask.
Apodization is yet another simple technique to modify the beam shape and thereby spot shape.
Published U.S. patent application in the name of Baird et al., U.S. 2002/0005396 A1, discloses a UV laser system wherein an optics module is provided to enhance shape quality of laser beams.
Sun et al. in U.S. Pat. No. 5,265,114 describe a method and system for selectively laser processing a target structure of one or more materials of a multi-material, multi-layer device.
Sun et al. in U.S. Pat. No. 6,057,180 describe a method for severing electrically conductive links with ultraviolet laser output.